Fluid Simulation: an object set as outflow doesn't remove all the "water" mesh
It does seem to be a bug and should be reported. Though it seems that there is currently no one maintaining the elbeem fluid sim code.

A possible work around would be to use a boolean modifier.

http://pasteall.org/blend/index.php?id=46863

Note that you'll need to bake first. I left the boolean modifier off (eye icon not checked) to illustrate that the boolean modifier removes the resultant fluid mesh, so you may need to tweak the boolean object to catch any particles/mesh that happen to move above the boolean object later in the animation.

Also, using a boolean modifier with a fluid sim mesh will likely really drag down the UI responsiveness.